Puzzling symptoms:
Over most internet connections the laptop has no trouble connecting to my IMAP email server. Everything works fine.
At my apartment my desktop has no trouble connecting to my IMAP email server. Everything works fine.
At my apartment my laptop (with nothing at at all changed in its configuration)
- authenticates and connects to my IMAP server just fine
- but it does not succeed in downloading messages from the same email server.
At my apartment my laptop (via a VPN connection to the server)
- has no trouble connecting to my IMAP email server and downloading messages

I recorded log files from thunderbird, the email client that I use, for the two cases.
The problem comes after the line:
3076[241d0e0]: 278e008:gg-office.harvard.edu:S-INBOX:SendData: 11 UID fetch 1:* (FLAGS)
The case that succeeds in downloading email has the next line:
6828[27a8c00]: ReadNextLine [stream=2538b20 nb=47 needmore=0]
The case that fails has instead as its next line:
3076[241d0e0]: ReadNextLine [stream=2520e88 nb=0 needmore=1]

Longer sections of the logfiles when thunderbird succeeds and fails follow:

This is the relevant section from the log of the IMAP server showing a successfull IMAP connection being made.
An unsuccessful attempt (below) is identical until after the space that I inserted in the following record.